streamline copyin/copyout code in usertests

fix bugs in read/write return values when there's an error

kernel/console.c

@@ -76,7 +76,7 @@ consolewrite(int user_src, uint64 src, int n)
   }
   release(&cons.lock);
 
-  return n;
+  return i;
 }
 
 //

kernel/fs.c

@@ -472,7 +472,7 @@ readi(struct inode *ip, int user_dst, uint64 dst, uint off, uint n)
     }
     brelse(bp);
   }
-  return n;
+  return tot;
 }
 
 // Write data to inode.

kernel/pipe.c

@@ -96,7 +96,7 @@ pipewrite(struct pipe *pi, uint64 addr, int n)
   }
   wakeup(&pi->nread);
   release(&pi->lock);
-  return n;
+  return i;
 }
 
 int